PCI: Document connection between pci_power_t and hardware PM capability

The dev.pme_support field, pci_pm_init(), pci_pme_capable(), and
pci_raw_set_power_state() depend on the fact that the pci_power_t values
(PCI_D0, PCI_D1, etc.) match the definition of the Capabilities PME_Support
and the Control/Status PowerState fields in the Power Management capability
(see PCI Bus Power Management spec r1.2, sec 3.2.3).

Add a note to this effect at the pci_power_t typedef.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>